The present invention relates to targets for in-line lithography process control. More particularly these targets are used in the monitoring of lithographic and etch processes used in microelectronics manufacturing.
Lithography has a broad range of industrial applications including the manufacture of semiconductors, displays, micro-machines and disk heads.
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light to a photoresist film on a substrate. Those segments of the absorbed aerial image, whose energy exceeds a threshold energy of chemical bonds in the photoactive component of the photoresist material, create a latent image in the photoresist. In some photoresist systems the latent image is formed directly by the photoactive component; in others the photochemical interaction first generates acids which react with other photoresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of photoresist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the photoresist film.
The principal determinant of the photoresist image is the surface on which the exposure energy equals the photoresist threshold energy in the photoresist film. xe2x80x9cExposurexe2x80x9d and xe2x80x9cfocusxe2x80x9d are the primary variables that control the shape of this surface. Exposure, set by the illumination time and intensity, determines the average energy of the aerial image per unit area. Local variations in exposure can be caused by variations in substrate reflectivity and topography. Focus, set by the position of the photoresist film relative to the focal plane of the imaging system, determines the decrease in modulation relative to the in-focus image. Local variations in focus can be caused by variations in substrate film thickness and topography.
Generally, because of the variations in exposure and focus, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable range. The importance of such monitoring increases considerably as the resolution limit, usually defined as minimum feature size resolvable, of the lithographic process is approached. The patterns being developed in semiconductor technology are generally in the shape of lines both straight and with bends, having a length dimension equal to and multiple times the width dimension. Since the width dimension is the minimum dimension of the patterns, it is the width dimension that challenges the resolution limits of the lithographic process. The term bias is used to describe the change in a dimension of a feature from its nominal value. Usually the bias of interest is the change in the smallest of the dimensions of a given feature.
As the lifetime of optical lithography is extended, the microelectronic industry pushes the 248 nanometer tools well beyond their intended specifications. Throughout the industry, smaller line widths and tighter tolerances are achieved by exploiting image enhancement techniques like phase-shift masks, off-axis illumination, and optical proximity correction. These enhancements have been sufficient to demonstrate process capability, but cannot guarantee routine manufacture of line widths below 0.2 microns. The lithography tools run well beyond specifications under manufacturing conditions, rendering historical tool controls inadequate. While wafer flatness, underlying film stacks, resist uniformity, and lithographic tool conditions all influence the final resist images, both focus and dose are tool process conditions which can be adjusted easily to control the critical resist image dimension (CD). A good control scheme should integrate both focus and dose adjustments to deliver the desired resist image.
Therefore the need for wafer-level understanding and control of dose and defocus stresses the importance of detecting dose and focus shifts. What is needed is a better way of detecting the subtle shifts in both due to line-shortening affects. What the inventors have discovered is that changing the shape of line-ends in the process control targets used can influence the sensitivity of the detection of either dose or focus shifts. The inventors change the bias of the line-ends used in the control targets to either increase or decrease the sensitivity to focus or dose. These line and shapes designs are referred to as anchors and noses.